Week 4: Project - Transient simulation of flow over a throttle body
A butterfly valve is used for regulating the fluid passage by opening and closing depending upon the requirement. The butterfly valve disc is placed on the center of the pipe and a rod passes through the disc to an actuator on the outside of the valve.
Rotating the actuator turns the disc either parallel or perpendicular to the flow . The disc is always present in the pipe and hence there is always a pressure drop even when the disc is open. It only rotates by a maximum of 90 degrees. When it completely rotates by 90 degrees, the fluid is completely blocked from flowing and when the disc is parallel to the direction of flow, the fluid completely flows without obstruction.
Setup and boundary conditions:
The setup and boundary conditions are almost the same as steady state simulation with just a few changes in the bounadry conditions for the throttle valve.
Boundary conditions for throttle:
The wall motion type should be made rotating and the surface movement should be moving. The rotation center and rotation origin have to specified for the throttle valve.
For the rotation rate we go to the use file option and enter the angle of rotation at the time instance of the simulation.
Observations:
The above is the animation of the mesh of transient simulation of the throttle.
Mass flow rates at inlet and outlet
The mass flow rate at inlet is equal to the mass flow rate at the outlet for every second of the simulation indicating that irrespective of the throttle position, the mass flow rate at the outlet is maintained equalto the incoming mass flow rate indicating that as throttle closes,the velocity although it is decreasing at one side of the throttle, it is proportionally increasing on the other side of the throttle in order to maintain the mass flow rate throughout the pipe.
Total pressure at inlet and outlet
pressure at rotated position that is facing towards the inlet
The pressure value is maximum after the throttle has rotated by 25 degrees indicating that the fluid flow is being restricted due to the movement of throttle. If the rotation would have been 90 degrees then the entire surface would have appeared red in color and have the highest pressure along the entire throttle.
The maximum pressure in this case is 1.5e5 pascal.
pressure at 25 degree rotated position on outlet facing side of the throttle
This is the backside view of the throttle at time 0.04 seconds ( at 25 degrees rotation). The pressure at the backside of the throttle is least in this position for the entire duration of the simulation. If the rotation would have been 90 degrees, then the entire surface would appear blue in color.
The minimum pressure value throughout the roation of the nozzle is 4.2e-4.
pressure at the end of the simulation time.
There are few zones where there is some good amout of pressure and the least pressure in this position is 5.6e-4. There would certainly be a little amount of pressure as the throttle is in the pathway of the elbow throughout the entire time of the simulation.
The animation of the pressure developed in the elbow throttle body in case of transient simulation.
Velocity at rotated position and on the throttle surface facing towards the inlet
The velocity values are lowest in this position of the throttle throughout the entire simulation. This is because the pressure values are the highest in this position and according to the governing equations of fluid mechanics, pressure and velocity are inversely proportional to each other.
Velocity at rotated position and on the throttle surface facing towards the outlet
There are certain areas on the backside of the nozzle that have high velocity. This high velocity is because there are low pressures on this side. Also velocity increases to a maximum of 3.5e2 m/s in order to maintain the mass flow rate of air that is flowing.
The animation for the velocity along with velocity vectors for transient case.
In this plot it can be seen that the red colored velocity vectors are emerging indicating the increase in velocity on this side of the nozzle.
Conclusions: In this simulation we observe that in certain instances of time, there are several low pressure regions on the surface. Normally low pressure regions are responsible for cavitation in fluids and specifically liquids. However since we have considered air for the simulation, the effect of cavitation is minimal or almost null. Through this simulation it can be concluded that pressure and velocity are inversely proportional to each other according to the governing equation of fluid mechanics and also the continuity and momentum governing equations of CFD hold good with the above transient simulation